Liquid crystal elements are widely used as e.g. displays in a display field and as e.g. diffraction elements and phase plates in an optical pickup field. These elements are required not only to have various properties such as a high contrast ratio, a wide view angle, a wide operating temperature range, a high-speed response, a low operating voltage and chemical stability, but also to decrease the thickness and size of the elements. Therefore, important physical properties are refractive index anisotropy, dielectric constant anisotropy, electrical conductivity, etc. of a liquid crystal material to be used for the liquid crystal elements, and in particular there is an increasing demand for a material with a large value of refractive index anisotropy.
For example, a product of a value of refractive index anisotropy (Δn) of a liquid crystal material and a thickness (d1) of a cell is required to be constant in the display elements. The recent display elements adopt a method of attaining good-quality display without domains by reducing d1, and a liquid crystalline compound with a large Δn value is needed for adjusting the Δn value of the liquid crystal material to an optimum value. Furthermore, because a response time can be reduced by decreasing the d1 value, a liquid crystalline compound with a large Δn value is extremely useful for producing a liquid crystal display element with a high response speed and good display quality.
Moreover, in the optical pickup field, it is proposed to realize a polarization diffractive element with high light utilization efficiency by forming a lattice-like rectangular structure on a transparent substrate and filling the structure with a liquid crystal material. When such a polarization diffractive element satisfies λ/2=Δn·d2 where d2 is a height of the lattice, Δn is a value of the refractive index anisotropy of the liquid crystal material, and λ is a wavelength of light to be used, the ±1st-order diffraction efficiency becomes maximum. In recent years, with downsizing of optical head apparatus, a lattice pitch p of the polarization diffractive element is becoming smaller and smaller. However, as an aspect ratio d2/p of the lattice becomes larger in accordance with the decrease in the pitch p, deviation from an ideal lattice-like structure becomes larger, which causes a problem of decrease in the diffraction efficiency. It is thus necessary to decrease the lattice height d2 and thereby decrease the aspect ratio. Namely, in order to satisfy both of the small aspect ratio and good diffraction efficiency, there is a demand for a liquid crystalline compound with a large value of refractive index anisotropy.
Compounds below are proposed heretofore as liquid crystalline compounds having a large value of refractive index anisotropy (in the formulae, R3 and R4 are each independently an alkyl group with a carbon number of from 2 to 12, an alkoxy group with a carbon number of from 2 to 12, a fluorine atom, a chlorine atom, a cyano group or a nitro group, and X3 is a hydrogen or fluorine atom).
(1) Tolan derivatives represented by the formula (A) below (cf. Patent Document 1)
(2) Butadiyne derivatives represented by the formula (B) below (cf. Non-patent Document 1)
(3) Stilbene derivatives represented by the formula (C) below (cf. Non-patent Document 2)
(4) Difluorostilbene derivatives represented by the formula (D) below (cf. Patent Document 2)
(5) Enyne derivatives represented by the formula (E) below (cf. Patent Document 3)
(6) Hexenediyne derivatives represented by the formula (F) below (cf. Patent Document 4)
    Patent Document 1: JP-A-1-502823    Patent Document 2: JP-A-3-294386    Patent Document 3: JP-A-6-312946    Patent Document 4: JP-A-7-304694    Non-patent Document 1: “Molecular Crystal Liquid Crystal”, 1978, vol. 48, p. 175    Non-patent Document 2: “Liquid Crystal”, 1993, vol. 15, p. 529-540